As a conventional electronic component socket, there is an IC socket which accommodates an electronic component such as an integrated circuit (IC) package for conducting a performance test such as a burn-in test on the IC package. Such a conventional IC socket includes contact pins each having a plated metal surface as electrical contact terminals for electrically connecting the IC package to a wiring board. The IC package includes connection terminals to be in electrical contact with the contact pins of the IC socket. In some IC packages, each connection terminal is formed using what is called a “lead-free solder” which contains tin as a principal ingredient but no lead.
Recently, an IC package has been required to withstand usage environments involving higher temperature conditions such as in a control unit located in an engine compartment. In line with this trend, the test temperatures set in burn-in tests also tend to be higher (150° C. or higher, for example). When a burn-in test is conducted at a higher test temperature, tin contained in each connection terminal of the IC package dissolves and diffuses with higher rate into the metal plating layer formed on the surface of the corresponding contact pin of the IC socket, thus forming an alloy along the interface with higher rate. Here, when the IC package is taken out of the IC socket with the connection terminal adhered to the contact pin, fracture occurs in such an alloy layer formed along the interface between the plated metal layer of the contact pin and the connection terminal of the IC package, and the plated metal layer partially peels off with the connection terminal. Thus, when the test temperature becomes higher and the rate of alloying increases, an alloy layer is formed at a higher rate and the plated metal layer is reduced more quickly.
In a known solution for this, for each contact pin, a plated palladium-nickel alloy layer is formed on a plated nickel layer that serves as the underlying layer, and a plated Ag layer, in which tin diffuses more slowly than in the plated palladium-nickel alloy layer, is formed on the outer side of the plated palladium-nickel alloy layer so as to serve as the outermost surface layer (see Patent Document 1, for example).
In such a contact pin, when tin contained in each connection terminal of the IC package dissolves and diffuses into the plated metal layer of the corresponding contact pin, only an extremely thin silver-tin alloy layer is formed at an initial stage along the interface between the plated metal layer of the contact pin and the connection terminal of the IC package. Accordingly, such a contact pin can minimize the portion of the plated silver layer that peels off with the connection terminal of the IC package when the IC package is taken out of the IC socket. This delays diffusion of tin into the plated palladium-nickel alloy layer, located further inside, thus reducing the rate of reduction of the plated palladium-nickel alloy layer that is caused when the plated palladium-nickel alloy layer is partially alloyed with tin and peels off with the connection terminal of the IC package.